A typical automotive engine management system (EMS) includes numerous electromagnetic actuators that are controlled by an electronic control module (ECM). One technique for performing actuator control involves current modulation to achieve a desired actuator current to operate an armature of the actuator. In various applications, it is desirable that the EMS measure an actuator current on a high-side of the load, i.e., actuator coil, when the high-side of the load is either at EMS voltage or, in some applications, boosted EMS voltage. Measurement of the current provided to the load can become problematic, as the measurement must take place in the presence of high common mode voltage. A classical method of current measurement involves the insertion of a known resistance in the current path and the measurement of the resulting voltage drop across the resistance. However, for systems that have high actuator currents, it is generally desirable to utilize a relatively low value resistor to achieve acceptable component power dissipations.
Traditionally, due to economic considerations, a current mirror has been utilized to make load current measurements. Unfortunately, current mirrors are notoriously inaccurate, due to variations in transistor parameters. Further, in a system that implements a current mirror to measure a high-side load current, the system may suffer from degraded actuator performance in that the actuator current may not be controlled within desired parameters. Additionally, such systems typically use a metal-oxide semiconductor field-effect transistor (MOSFET) to act as a low-side switch, i.e., a switch placed between a low-side of the load and ground. MOSFET drive circuits, associated with the MOSFET, typically exhibit overshoot and undershoot errors due to drive circuit component tolerances. These current overshoot and undershoot errors may also lead to degraded actuator performance.
Historically, ECMs have been factory calibrated to provide a desired actuator current to a given actuator. However, in such factory calibrated systems, the calibration is fixed for the life of the product and is valid only if the ECM and actuator performance do not drift over the product lifetime. Further, the calibration is dependent on accurate correlation between the load, used during calibration, and the actual load of the end application. It should be appreciated that this necessitates tight tolerance requirements on the actuator, and accordingly, increased actuator cost. Additionally, ECM components will inevitably drift over the life of the system, which will result in degraded actuator performance. Given that the calibration is performed at a single ECM temperature, the calibration also does not necessarily mitigate errors over the operational temperature range of the ECM. In addition, traditional calibration processes have increased manufacturing capital equipment requirements and manufacturing cycle times and have, thus, led to increased product cost.
What is needed is a technique for calibrating a current control circuit that provides adequate current measurement accuracy, without the need for factory calibration using external equipment. It would also be desirable if the calibrated current control circuit was capable of compensating for drift in component tolerances over the life of the product.